Saturday, February 27, 2010

When a Magnitude 6.9 Earthquake is an Aftershock....

There wasn't a whole lot of information when I wrote about the 8.8 earthquake in Chile in the earliest hours of the morning. Information is starting to flood in now from many sources, and my first impression is media coverage is a bit better than it was for the Haiti quake and other earlier events. When I say "better", I don't mean more cameras, I mean evidence of an effort by the reporters to educate themselves about the basic science of earthquakes.

My first thought is for the people who have been affected, both in Chile and across the Pacific basin. A quake this big is going to have huge impacts. The aftershocks are going to be an issue: my USGS notifications are set for any quakes bigger than 5.5 and there have been 11 of them so far, with the largest at 6.9. For perspective, 6.9 is the size of the Loma Prieta earthquake here in California from 1989. It killed 4 dozen people and did around $10 billion in damage. The quake in Haiti was a somewhat larger 7.0. Aftershocks are going to continue for a long time, over a region about 600 miles long (the size of the fault zone that shifted in this quake).

The tsunami is a huge threat. The size of a tsunami in any particular place is dependent on many factors, including the size of the earthquake (which was colossal), how the quake transferred energy to the ocean water (unknown to me at this point), and the shape and depth of the coastline where the tsunami strikes. All I can say to my Pacific Basin friends is that when the Civil Defense folks set off the sirens, take it seriously! Hawaii in particular has a tragic history with tsunamis, and there has been a vast amount of coastal development since the last major tsunami (in 1960, also caused by an earthquake in Chile). There will be a big problem of people wandering down to the beach to watch the tsunami, which can only be described as an act of idiocy. Get to higher ground or higher floors. If the warnings are inaccurate, you've only lost a bit of time, but if they are accurate, your life will be saved. Don't go to the coast until the "all clear" is given, because there will be more than one surge of water. You can't swim your way out of a tsunami; they are one of the most dangerous of geologic events, as the Indonesian tsunami of 2004 demonstrated.


Gaelyn said...

Thanks for the update Garry. Keep 'em coming, please. No TV here.

Edie Howe said...


Thanks for the information on the Chilean earthquakes.

One of my favorite sites is the USGS Earthquake page, and I've just subscribed to their RSS feed notification (mag 2.5>). I've noticed a lot of Ring of Fire quakes over the last few hours; Alaska, Japan, California, Chile, and some out by Australia. Just how common is it for this section of the planet to have earthquakes in such succession?

Have you ever seen a horse or a dog shiver its skin to discourage flies? I sometimes think the planet is doing the same thing.

BTW, I'm fighting an urge to go to McWay Cove to see the tsunami roll northward; I keep telling myself I won't get there in time, and I don't have enough money for gas!

Thanks for the info,
Hunkered down in Yosemite with fog and rain and sleet and snow.

Garry Hayes said...

There are always dozens of earthquakes across the Pacific rim every day, so there is nothing unusual about the activity, you may even eventually find you want to put the magnitude on your notifications up to 4.5 or 5.5 because of the sheer number of e-mails you will get. At the moment I am getting info on world events greater than 5.5, and anything larger than 4.5 in California. My e-mail doesn't get flooded that way. California alone has had more than 700 quakes of magnitude 1 and above this week.

Crysta said...

Good thing you never set it to 5 or you woulda had around 80 for just chile emails.

Seems horrible, I just get the worst feeling watching the aftershocks pile up. Hope in the end it won't be as bad as I imagine!
Good luck Chile!